1. Field of the Invention
The present invention relates to a tellurite glass as a glass material for an optical fiber and an optical waveguide, and in particular a broadband optical amplification medium using the tellurite glass which is capable of working even at wavelengths of 1.5 xcexcm to 1.7 xcexcm. The present invention also relates to a broadband optical amplifier and a laser device using the broad band optical amplification medium. Furthermore, the present invention relates to a method of splicing a non-silica-based optical fiber and a silica-based optical fiber reliably with the characteristics of low fiber-loss and low reflection.
2. Description of the Related Art
The technology of wavelength division multiplexing (WDM) has been studied and developed for expanding transmission volume of optical communication systems and functionally improving such systems. The WDM is responsible for combining a plurality of optical signals and transmitting a combined signal through a single optical fiber. In addition, the WDM is reversibly responsible for dividing a combined signal passing through a single optical fiber into a plurality of optical signals for every wavelength. This kind of transmitting technology requires a transit amplification just as is the case with the conventional one according to the distance of transmitting a plurality of optical signals of different wavelengths through a single optical fiber. Thus, the need for an optical amplifier having a broad amplification waveband arises from the demands for increasing the optical signal""s wavelength and the transmission volume. The wavelengths of 1.61 xcexcm to 1.66 xcexcm have been considered as appropriate for conserving and monitoring an optical system, so that it is desirable to develop an optical source and an optical amplifier for that system.
In recent years, there has been considerable work devoted to research and development on optical fiber amplifiers that comprise optical fibers as optical amplification materials, such as erbium (Er) doped optical fiber amplifiers (EDFAs), with increasing applications to the field of optical communication system. The EDFA works at a wavelength of 1.5 xcexcm where a loss of silica-based optical fiber decreases to a minimum, and also it is known for its excellent characteristics of high gain of 30 dB or more, low noise, broad gain-bandwidth, no dependence on polarized waves, and high saturation power.
As described above, one of the remarkable facts to be required of applying the above EDFA to the WDM transmission is that the amplification waveband is broad. Up to now, a fluoride EDFA using a fluoride glass as a host of an erbium-doped optical fiber amplifier has been developed as a broad amplification band EDFA.
In U.S. Pat. Nos. 3,836,868, 3,836,871, and 3,883,357, Cooley et al. discloses the possibility of laser oscillation to be caused by tellurite glass containing an rare earth element. In this case, however, Cooley et al. have no idea of forming tellurite glass into an optical fiber because there is no description concerned about the adjustment of refractive index and the thermal stability of tellurite glass to be required for that formation.
In U.S. Pat. No. 5,251,062, Snitzer et al. insists that tellurite glass play an important role in extending the EDFA""s amplification band and it should be formed into a fiber which is absolutely essential to induction of an optical amplification. Thus, they disclose the allowable percent ranges of ingredients in tellurite-glass composition in a concretive manner. The tellurite-glass composition includes a rare earth element as an optically active element and can be formed into a fiber. More specifically, the tellurite-glass composition of Snitzer et al. is a ternary composition comprising TeO2, R2O, and QO where R denotes a monovalent metal except Li and Q denotes a divalent metal. The reason why Li is excluded as the monovalent metal is that Li depresses thermal stability of the tellurite-glass composition.
In U.S. Pat. No. 5,251,062, furthermore, Snitzer et al. make a comparative study of fluorescence erbium spectra of silica and tellurite glass and find that the tellurite glass shows a broader erbium spectrum compared with that of the silica glass. They conclude that the ternary tellurite glass composition may allow a broadband amplification of EDFA and an optically active material such as praseodymium or neodymium may be added in that composition for inducing an optical amplification. In this patent document, however, there is no concrete description of the properties of gain, pump wavelength, signal wavelength, and the like which is important evidence to show that the optical amplification was actually down. In other words, U.S. Pat. No. 5,251,062 merely indicate the percent ranges of ingredients of ternary tellurite glass composition that can be used in an optical fiber.
Furthermore, Snitzer et al. show that thermal and optical features of various kinds of tellurite glass except of those described in U.S. Pat. No. 5,251,062 in a technical literature (Wang et al., Optical Materials, vol. 3 pages 187-203, 1994; hereinafter simply referred as xe2x80x9cOptical Materialsxe2x80x9d). In this literature, however, there is also no concrete description of optical amplification and laser oscillation.
In another technical literature (J. S. Wang et al., Optics Letters, vol. 19 pages 1448-1449, 1994; hereinafter simply referred as xe2x80x9cOptics Lettersxe2x80x9d) published in right after the literature mentioned above, Snitzer et al. show the laser oscillation for the first time caused by using a single mode optical fiber of neodymium-doped tellurite glass. The single mode fiber comprises a core having the composition of 76.9% TeO2-6.0% Na2O-15.5% ZnO-1.5% Bi2O3-0.1% Nd2O3 and a clad having the composition of 75% TeO2-5.0% Na2O-20.0% ZnO and allows 1,061 nm laser oscillation by 81 nm pumping. In this literature, there is no description of a fiber loss. In Optical Materials, on the other hand, there is a description of which the loss for an optical fiber having a core composition of Nd2O3-77% TeO2-6.0% Na2O-15.5% ZnO-1.5% Bi2O3 and a clad composition of 75% TeO2-5.0% Na2O-20.0% ZnO (it is deemed to be almost the same composition as that of Optics Letters) is 1500 dB/km at a wavelength of 1.55 xcexcm (see FIG. 1 that illustrates a comparison between 4I13/2 to 4I15/2 Er3+ emission in tellurite glass and 4I13/2 to 4I15/2 Er3+ emission in fluoride glass). The core composition of this optical fiber is different from that of a ternary composition disclosed in U.S. Pat. No. 5,251,062 because the former includes Bi2O3. It is noted that there is no description or teach of thermal stability of Bi2O3-contained glass composition in the descriptions of Optics Letters, Optical Materials, and U.S. Pat. No. 5,251,062 mentioned above.
However, the fluoride based EDFA has an amplification band of about 30 nm which is not enough to extend an amplification band of optical fiber amplifier for the purpose of extending the band of WDM.
As described above, tellurite glass shows a comparatively broader fluorescence spectral band width, so that there is a possibility to extend the amplification band if the EDFA uses the tellurite glass as its host. In addition, the possibility of producing a ternary system optical fiber using the composition of TeO2, R2O, and QO (wherein R is a univalent metal except Li and Q is a divalent atom) has been realized , so that laser oscillation at a wavelength of 1061 nm by a neodymium-doped single mode optical fiber mainly comprising the above composition has been attained. In contrast, EDFA using tellurite glass is not yet realized. Therefore, we will describe the challenge to realize a tellurite-based EDFA in the following.
First, the difference between the objective EDFA and the neodymium-doped fiber laser (i.e., the difference between 1.5 xcexcm band emission of erbium and 1.06 xcexcm band emission of neodymium in glass) should be described in detail.
An optical transition of the objective EDFA is shown in FIG. 2 where three different energy levels are indicated by Level 1, Level 2, and Level 3, respectively. For attaining an objective induced emission from Level 2 to Level 1, a population inversion between Level 1 and Level 2 is done by pumping from Level 1 to Level 3 and then relaxing from Level 3 to Level 2. This kind of the induced emission can be referred as a three-level system.
In the case of the neodymium, as shown in FIG. 3, a four-level system can be defined that a final level of the induced emission is not a ground level but a first level (Level 1) which is higher than the ground level. Comparing the three-level system with the four-level system, the former is hard to attain the population inversion so that an ending level of the induced emission is in a ground state. Accordingly, the three-level system EDFA requires enhanced optically pumping light intensity, and also the fiber itself should be of having the properties of low-loss and high xcex94n. In this case, the high xcex94n is for effective optically pumping.
Secondly, we will briefly described that an amplification band cannot be extended even if it is possible to perform an optical amplification when a transmission loss of fiber is large.
Wavelength dependencies of the silica-based EDFA and the tellurite-based EDFA are illustrated in FIG. 4. As shown in the figure, it can be expected that the tellurite-based EDFA will attain a broadband optical amplification broader than that of the silica-based EDFA. Comparing with the silica-based glass and the non-silica-based glass, a transmission loss at a communication wavelength of the latter is substantially larger than that of the former. In the optical fiber amplifier, therefore, the loss leads to a substantial decrease in gain.
As schematically shown in FIG. 5, if the loss is comparatively small, the amplification band of tellurite glass is close to the one shown in FIG. 4. If the loss is comparatively large, on the other hand, the amplification band of tellurite glass is narrowed.
In recent technical investigations on WDM transmission, by the way, it has been made attempts to speed up transmission through one channel for increasing transmission capacity. To solve this problem, it is necessary to optimize the chromatic dispersion characteristics of the Er-doped optical fiber. Up to now, however, no attention have been given to that characteristics.
For the tellurite glass, a wavelength at which a chromatic dispersion value takes zero is in the wavelengths longer than 2 xcexcm. In the case of a high NA (Numerical Aperture) fiber to be used in EDFA, a chromatic dispersion value is generally xe2x88x92100 ps/km/nm or less at 1.55 xcexcm band. Thus, a chromatic dispersion of a short optical fiber of about 10 m in length also takes the large value of xe2x88x921 ps/nm or less.
For the use of tellurite EDFA in long-distance and high-speed WDM transmission, therefore, it is need to bring the chromatic dispersion close to zero as far as possible. As described above, however, as the material dispersion value of tellurite glass takes the value of zero at wavelengths of 2 xcexcm and over. Therefore, the tellurite-based optical fiber cannot utilize the technique adopted in the silica-based optical fiber that brings the chromatic dispersion value at 1.55 xcexcm band close to zero by optimizing the construction parameters of the fiber.
Furthermore, the tellurite-based optical fiber can be used as a host of praseodymium (Pr) for 1.3 xcexcm band amplification. As described above, however, the tellurite-bade optical fiber has a large chromatic dispersion value as the absolute value. In the case of amplifying a high-speed optical signal by using the tellurite-based optical fiber, a distortion of pulse wavelength can be induced and thus the chromatic dispersion value should be corrected for. If not, the use of tellurite glass in an optical communication system falls into difficulties.
Next, an optical-fiber splicing between a non-silica-based optical fiber and a silica-based optical fiber will be described below.
For using the above non-silica-based topical fiber such as a tellurite optical fiber as an optical amplification or nonlinear optical fiber, there is a necessity to connect to a silica-based optical fiber to form the junction between these fibers with low-loss and low reflection. However, these fibers have their own core refractive indexes which are different from each other. If these fibers are connected together as shown in FIGS. 6 and 7, a residual reflection can be observed so that the junction appropriately adaptable to practical use cannot be implemented. In FIGS. 6 and 7, reference numeral 1 denotes a non-silica-based optical fiber, 2 denotes a silica-based optical fiber, 5 denotes an optical binder, and 6 denotes a binder. In FIG. 6, furthermore, there is no optical binder applied on a boundary surface between the fibers. As shown in FIG. 8, therefore, the existence of residual reflection between the silica-based optical fibers 2a, 2b and the non-silica-based optical fiber 1 degrades the quality of signal because of a ghost (which acts as noise) due to a reflected signal on the connected ends of the fibers. Therefore, the connected portion between those fibers require xe2x88x9260 dB or over as a residual reflection factor for an optical amplifier (see Takei et al. xe2x80x9cOptical Amplifier Modulexe2x80x9d, Okidenki Kaihatu, vol. 64, No. 1, pp 63-66, 1997). For example, a zirconium-doped fluoride fiber, an indium-doped fluoride fiber, chalcogenide glass fiber (i.e., glass composition: Asxe2x80x94S), and a tellurite glass fiber have their own core""s refractive indexes of 1.4 to 1.5, 1.45 to 1.65 and 2.4 and 2.1, respectively, depending on the variations in their glass compositions. If one of those fibers is connected to a silica based optical fiber (core""s refractive index is about 1.50 or less), a return loss R can be obtained by the formula (2) described below. In this case, the unit of R is dB and the residual reflective index is expressed in a negative form while the return loss is expressed in a positive form as an absolute value of the residual reflective index. The return loss can be obtained by the equation (1) below.                     R        =                              "LeftBracketingBar"                          10              ⁢                              xe2x80x83                            ⁢              log              ⁢                              {                                                      [                                                                  (                                                                              n                            NS                                                    -                                                      n                            S                                                                          )                                                                    (                                                                              n                            NS                                                    +                                                      n                            S                                                                          )                                                              ]                                    2                                }                                      "RightBracketingBar"                    ⁢                      xe2x80x83                    ⁢                      (            dB            )                                              (        1        )            
where nNS and nS are core""s refractive indexes of silica and non-silica optical fibers, respectively. The return loss between the silica-based optical fiber and the zirconium-doped fluoride optical fiber, indium-doped optical fluoride fiber, chalcogenide glass fiber (i.e., glass composition: As-S), or tellurite glass fiber is 35 dB or more, 26 dB or more, 13 dB, or 16 dB, respectively. In the case of Zr-based and In-based fluoride optical fiber, the return loss can be increased (while the residual reflection coefficient can be decreased) by bringing their refractive indexes to that of the silica-based optical fiber""s core by modifying their glass compositions, respectively. However, the modification of glass composition leads to the formation of practical optical fiber under the constraint that the glass composition should be precisely formulated in the process of forming a fiber in a manner which is consistent with an ideal glass composition for the process of forming a low-loss fiber). A coupling between the silica-based optical fiber and the non-silica-based optical fiber has the following problems. That is, conventional fusion splicing procedures cannot be applied because of the difference in softening temperatures of both fibers (i.e., 1,400xc2x0 C. for the silica-based optical fiber and less than 500xc2x0 C. for the non-silica-based one); the conventional optical connector coupling technologies cannot be applied because there is no appropriate coupling method for the non-silica-based optical fiber; and so on. Thus, a general coupling method for coupling the Zr-based or In-based optical fiber to the silica-based optical fiber without depending on its glass composition has been demanded. In addition, a general coupling method for reliably coupling the chalcogenide glass optical fiber or the tellurite optical fiber to the silica-based optical fiber with a low-loss and low-reflection.
One of the conventional coupling technologies for solving such problems, Japanese Patent Application Laying-open No. 6-27343, is illustrated in FIGS. 9 and 10. In this technology, a non-silica-based optical fiber 1 and a silica-based optical fiber 2 are held in housings 7a and 7b, respectively. The fibers 1, 2 are positioned in their respective V-shaped grooves on substrates 8a, 8b and fixed on their respective housings 7a, 7b by means of bonding agents 10a, 10b and fiber-fixing plates 9a, 9b. In addition, there is a dielectric film 18 applied on a coupling end of one of the housings for preventing a reflection to be generated by coupling the fibers together. The coupling between the non-silica-based optical fiber 1 and the silica-based optical fiber 2 are carried out by using an optical bonding agent 5 made of ultraviolet-curing region after adjusting the relative positions of the housings 7a, 7b so as to match their optical axes. At this moment, the coupling end of the housing 7a is perpendicular to the optical axis of the non-silica-based optical fiber and also the coupling end of the housing 7b is perpendicular to the optical axis of the silica-based optical fiber, so that if the reflection of light is occurred at a boundary surface of the coupling the reflected light returns in the reverse direction, resulting in a falloff in the return loss. Accordingly, the conventional technology uses the dielectric film 18 to reduce the reflection from the boundary surface of the coupling. However, the conventional coupling requires a precision adjustment to a refractive index of the optical biding agent 5 and a refractive index and thickness of the dielectric film 18. That is, their refractive indexes must satisfy the following equations (2) and (3) if a core""s refractive index of the non-silica-based optical fiber 1 is n1 and a core""s refractive index of the silica-based optical fiber 2 is n2. A refractive index of the optical binding agent 5 is adjusted to n1, while a refractive index and a thickness of the dielectric film 18 is adjusted to n1 and tf, respectively, so as to satisfy the following equations (2), (3).                               n          f                =                                            n              1                        ·                          n              2                                                          (        2        )                                          t          f                =                  λ                                                    n                1                            ·                              n                2                                      4                                              (        3        )            
wherein xcex is a signal wavelength (i.e., the wavelength to be used).
In the conventional technology, as described above, there is the need for precisely adjusting a refractive index of the optical binder 5 and a refractive index and thickness of the dielectric film 18 for constructing a coupling portion with the properties of low-reflection and low-loss by using the dielectric film 18. It means that the precise adjustments leads to difficulties in implementing a coupling between the fibers favorably with an improvement in yield.
A process of coupling two different optical fibers in accordance with another conventional technology, as shown in FIG. 11, comprises the steps of: holding an optical fiber 9a on a housing 7b and also holding an optical fiber 9a on a housing 7b; positioning these housings 7a, 7b in their right places so that a coupling end of the housing 7a that holds the optical fiber 19a and a coupling end of housing 7b that holds the optical fiber 19b are positioned with a xcex8-degree slant with respect to a direction perpendicular to the optical axes of the optical fibers 19a, 19b; and connecting the housing 7a and the housing 7b together after the positioning of the housings so as to concentrically adjust the optical axes in a straight line. The process is a so-called slant coupling method for realizing the coupling with low-reflection and low fiber-loss. However, this process is only applied to the fibers when their core refractive indexes are almost the same, so that it cannot be applied to the coupling between the non-silica-based optical fiber and the silica-based optical fiber which have different core refractive indexes with respect to each other.
A first object of the present invention is to provide a tellurite glass fiber of high xcex94n and low fiber-loss.
A second object of the present invention is to provide a tellurite glass fiber that includes the capability of realizing a broadband EDFA doped with an optically active rare earth element, which cannot be realized by the conventional glass compositions.
A third object of the present invention is to provide a broadband optical amplification medium that includes the capability of acting at wavelengths, especially from 1.5 xcexcm to 1.7 xcexcm, and also to provide an optical amplifier and a laser device which use such a medium and act at wavelengths in a broad range and have low-noise figures.
A fourth object of the present invention is to provide a general and practical technique of reliably coupling a non-silica-based optical fiber and a silica-based optical fiber or coupling optical fibers having different core refractive indexes with low fiber-loss and low reflection.
In a first aspect of the present invention, there is provided a tellurite glass as a glass material for an optical fiber or an optical waveguide, comprising:
0 less than Bi2O3xe2x89xa620 (mole %);
0xe2x89xa6Na2Oxe2x89xa635 (mole %);
0xe2x89xa6ZnOxe2x89xa635 (mole %); and
55xe2x89xa6TeO2xe2x89xa690 (mole %).
In a second aspect of the present invention, there is provided a tellurite glass as a glass material for an optical fiber or an optical waveguide, comprising:
1.5 less than Bi2O3xe2x89xa615 (mole %);
0xe2x89xa6Na2Oxe2x89xa635 (mole %);
0xe2x89xa6ZnOxe2x89xa635 (mole %); and
55xe2x89xa6TeO2xe2x89xa690 (mole %).
In a third aspect of the present invention, there is provided a tellurite glass as a glass material for an optical fiber or an optical waveguide, comprising:
0 less than Bi2O3xe2x89xa620 (mole %);
0xe2x89xa6Li2Oxe2x89xa625 (mole %);
0xe2x89xa6ZnOxe2x89xa625 (mole %); and
55xe2x89xa6TeO2xe2x89xa690 (mole %).
In a fourth aspect of the present invention, there is provided a tellurite glass as a glass material for an optical fiber or an optical waveguide, comprising:
0 less than Bi2O3xe2x89xa620 (mole %);
0xe2x89xa6M2Oxe2x89xa635 (mole %);
0xe2x89xa6ZnOxe2x89xa635 (mole %); and
55xe2x89xa6TeO2xe2x89xa690 (mole %), wherein
the M is at least two univalent metals selected from a group of Na, Li, K, Rb, and Cs.
In a fifth aspect of the present invention, there is provided a tellurite glass as a glass material for an optical fiber or an optical waveguide, comprising:
1.5 less than Bi2O3xe2x89xa615 (mole %);
0xe2x89xa6M2Oxe2x89xa635 (mole %);
0xe2x89xa6ZnOxe2x89xa635 (mole %); and
55xe2x89xa6TeO2xe2x89xa690 (mole %), wherein
the M is at least two univalent metals selected from a group of Na, Li, K, Rb, and Cs.
In a sixth aspect of the present invention, there is provided a tellurite glass as a glass material for an optical fiber or an optical waveguide, comprising:
0 less than Bi2O3xe2x89xa620 (mole %);
0xe2x89xa6Li2O3xe2x89xa625 (mole %);
0xe2x89xa6Na2Oxe2x89xa615 (mole %);
0xe2x89xa6ZnOxe2x89xa625 (mole %); and
60xe2x89xa6Teo2xe2x89xa690 (mole %).
In a seventh aspect of the present invention, there is provided a tellurite glass as a glass material for an optical fiber or an optical waveguide that contains erbium at least in a core, consisting of a glass composition that contains Al2O3.
In an eighth aspect of the present invention, there is provided a tellurite glass as a glass material for an optical fiber or an optical waveguide, wherein
the glass material has a composition of:
TeO2xe2x80x94ZnOxe2x80x94M2Oxe2x80x94Bi2O3xe2x80x94Al2O3 where M is at least one element.
In a ninth aspect of the present invention, there is provided a tellurite glass as a glass material for an optical fiber or an optical waveguide, comprising:
0 less than Bi2O3xe2x89xa610 (mole %);
0xe2x89xa6Li2O3xe2x89xa630 (mole %);
0xe2x89xa6ZnOxe2x89xa64 (mole %);
70xe2x89xa6TeO2xe2x89xa690 (mole %); and
0xe2x89xa6Al2O3xe2x89xa63 (mole %).
In a tenth aspect of the present invention, there is provided a tellurite glass as a glass material for an optical fiber or an optical waveguide, comprising:
0 less than Bi2O3xe2x89xa615 (mole %);
0xe2x89xa6Na2Oxe2x89xa630 (mole %);
0xe2x89xa6ZnOxe2x89xa635 (mole %);
60xe2x89xa6TeO2xe2x89xa690 (mole %); and
0xe2x89xa6Al2O3xe2x89xa64 (mole %).
Here, a concentration of the Bi2O3 may be:
4 less than Bi2O3 less than 7.
In an eleventh aspect of the present invention, there is provided an optical amplification medium comprised of an optical amplifier or an optical waveguide having a core glass and a clad glass, wherein
at least one of the core glass and the clad glass is made of the tellurite glass of one of the novel tellurite glasses described above.
In a twelfth aspect of the present invention, there is provided an optical amplification medium comprised of an optical amplifier or an optical waveguide having a core glass and a clad glass, wherein
the core glass is made of a tellurite glass a composition of:
0 less than Bi2O3xe2x89xa620 (mole %), preferably 1.5 less than Bi2O3xe2x89xa615 (mole %), or more preferably 4 less than Bi2O3xe2x89xa67;
0xe2x89xa6Na2Oxe2x89xa635 (mole %);
0xe2x89xa6ZnOxe2x89xa635 (mole %); and
55xe2x89xa6TeO2xe2x89xa690 (mole %), and
the clad is made of a tellurite glass having a composition selected from a group of:
a first composition including
5 less than Na2O less than 35 (mole %),
0xe2x89xa6ZnO less than 10 mole %), and
55 less than TeO2 less than 85 (mole %);
a second composition including
5 less than Na2O less than 35 (mole %),
10 less than ZnOxe2x89xa620 mole %), and
55 less than TeO2 less than 85 (mole %); and
a third composition including
0 less than Na2O less than 25 (mole %),
20 less than ZnOxe2x89xa630 mole %), and
55 less than TeO2 less than 75 (mole %).
Here, at least one of the core glass and the clad class may contain erbium or erbium and ytterbium.
At least one of the core glass and the clad glass may contain at least one selected from a group consisting of boron (B), phosphorus (P), and hydroxyl group.
At least one of the core glass and the clad glass may include an element selected from a group consisting of Ce, Pr, Nd, Sm, Tb, Gd, Eu, Dy, Ho, Tm, and Yb.
In a thirteenth aspect of the present invention, there is provided an optical amplification medium comprised of an optical amplifier or an optical waveguide having a core and a clad which are made of a glass material and at least the core contains erbium, wherein
the glass material consists of a tellurite composition that contains Al2O3.
In a fourteenth aspect of the present invention, there is provided an optical amplification medium comprised of an optical amplifier or an optical waveguide having a core and a clad which are made of a glass material and at least the core contains erbium, wherein
the glass material consists of a tellurite composition of:
TeO2xe2x80x94ZnOxe2x80x94M2Oxe2x80x94Bi2O3xe2x80x94Al2O3
xe2x80x83where
M is at least one alkali element.
Here, a cut-off wavelength may be in the range of 0.4 xcexcm to 2.5 xcexcm.
In a fifteenth aspect of the present invention, there is provided a laser device comprising an optical cavity and an excitation light source, wherein
at least one of optical amplification media in the optical cavity is one of the novel optical amplification media described above.
In a sixteenth aspect of the present invention, there is provided a laser device having a plurality of optical amplification media comprised of optical fibers that contain erbium in their cores and arranged in series, wherein
each of the optical amplification media is one of the novel optical amplification media described above.
In a seventeenth aspect of the present invention, there is provided a laser device having an amplification medium and an excitation light source, wherein
the amplification medium is one of the novel optical amplification media described above.
In an eighteenth aspect of the present invention, there is provided an optical amplifier having an optical amplification medium, an input device that inputs an excitation light and a signal light for pumping the optical amplification medium, wherein
the optical amplification medium is one of the novel optical amplification media described above.
In a nineteenth aspect of the present invention, there is provided an optical amplifier having a plurality of optical amplification media comprised of optical fibers that contain erbium in their cores and arranged in series, wherein
each of the optical amplification media is one of the novel optical amplification media described above.
In a twentieth aspect of the present invention, there is provided an optical amplifier having a tellurite glass as an optical amplification medium, comprising:
a dispersion medium which is placed on at least one position in the front of or at the back of the optical amplification medium, wherein
the dispersion medium compensates for dispersion of wavelengths by a value of chromatic dispersion that takes a plus or negative number opposite to a value of chromatic dispersion for the optical amplification medium.
Here, the optical amplification medium may be an optical waveguide made of a tellurite glass that contains a rare-earth element or a transition metal element.
The tellurite glass may consist of a composition selected from:
TeO2xe2x80x94ZnOxe2x80x94M2Oxe2x80x94Bi2O3;
TeO2xe2x80x94ZnOxe2x80x94M2Oxe2x80x94Bi2O3xe2x80x94Al2O3, and
TeO2xe2x80x94WO3xe2x80x94La2O3xe2x80x94Bi2O3xe2x80x94Al2O3 
where M is at least one alkali element.
The dispersion medium may be one selected from an optical fiber and a fiber-bragg-grating.
In a twenty-first aspect of the present invention, there is provided an optical amplifier having a plurality of stages of optical amplification portions that include erbium-doped optical fibers as their optical amplification media, wherein
a tellurite glass optical fiber is used as a material of the optical fiber in at least one of the optical amplification portions except one at the front thereof, and
an optical amplification portion positioned in front of the optical amplification portion using the tellurite glass optical fiber is comprised of an erbium-doped optical fiber, where
a product of an erbium-doping concentration and a fiber-length of the erbium-doped optical fiber is smaller than that of the tellurite glass fiber.
Here, the tellurite glass may consist of a composition selected from:
TeO2xe2x80x94ZnOxe2x80x94M2Oxe2x80x94Bi2O3; and
TeO2xe2x80x94ZnOxe2x80x94M2Oxe2x80x94Bi2O3xe2x80x94Al2O3,
where M is at least one alkali element.
A material of the optical amplification medium may be one selected from a group of a silica optical fiber, a fluoro-phosphate optical fiber, a phosphate optical fiber, and a calcogenide optical fiber, in addition to the tellurite optical fiber.
An optical fiber material except a tellurite optical fiber may be used as at least one optical amplification portion at any given stage up to the optical amplification portion using the tellurite glass fiber.
A product of an erbium-addition concentration and a fiber-length of at least one optical fiber, which is positioned at any given stage up to the optical amplification portion using the tellurite glass fiber, may be smaller than that of the tellurite optical fiber.
In a twenty-second aspect of the present invention, there is provided an optical amplifier using erbium-doped optical fibers as optical amplification media, comprising at least one arrangement configuration wherein
at least two tellurite optical fibers each having a different product of an erbium-doping concentration and a fiber-length are arranged in series so that the tellurite optical fiber having a smaller product of an erbium-addition concentration and a fiber-length is placed at the front stage up to the tellurite optical fiber having a larger product of an erbium-addition concentration and a fiber-length.
Here, the tellurite glass may consist of a composition selected from:
TeO2xe2x80x94ZnOxe2x80x94M2Oxe2x80x94Bi2O3; and
TeO2xe2x80x94ZnOxe2x80x94M2Oxe2x80x94Bi2O3xe2x80x94Al2O3,
where M is at least one alkali element.
In a twenty-third aspect of the present invention, there is provided an optical-fiber splicing structure for contacting a splicing end surface of a first housing in which an end of a first optical fiber is held and an splicing end surface of a second housing in which an end of a second optical fiber is held in a state of co-axially centering an optical axis of the first optical fiber and an optical axis of the second optical fiber, where at least one of the first optical fiber and the second optical fiber is a non-silica-based optical fiber, wherein
optical axes of the first and second optical fibers are held in the first and second housings respectively at angles xcex81 and xcex82 (xcex81 xe2x89xa0xcex82) from a vertical axis of a boundary surface between the splicing end surfaces, and a relationship between the angles xcex81 and xcex82 satisfies Snell""s law represented by an equation (4) at the time of splicing the first and second optical fibers:                                           sin            ⁢                          xe2x80x83                        ⁢                          θ              1                                            sin            ⁢                          xe2x80x83                        ⁢                          θ              2                                      =                              n            2                                n            1                                              (        4        )            
where n1 is a refractive index of the first optical fiber and n2 is a refractive index of the second optical fiber.
Here, the splicing end surface of the first optical fiber may be connected to the splicing end surface of the second optical fiber through an optical adhesive at the time of splicing the first and second optical fibers.
The splicing end surface of the first optical fiber and the splicing end surface of the second optical fiber may be kept in absolute contact with each other at the time of splicing the first and second optical fibers.
The first and second optical fibers may be non-silica-based optical fibers.
The non-silica-based optical fibers may be selected from Zr- or In-based fluoride optical fibers, chalcogenide optical fibers, and tellurite glass optical fibers.
The non-silica-based optical fibers may be selected from Zr- or In-based fluoride optical fibers, chalcogenide optical fibers, and tellurite glass optical fibers, and furthermore the non-silica-based optical fibers may be doped with a rare-earth element.
The first optical fiber may be a tellurite glass optical fiber, the second optical fiber may be a silica-based optical fiber, and the angle xcex81 may be of 8 or more degrees.
The first optical fiber may be a Zr-based fluoride optical glass fiber, the second optical fiber may be a silica-based optical fiber, and the angle xcex81 may be of 3 or more degrees.
The first optical fiber may be a In-based fluoride optical glass fiber, the second optical fiber may be a silica-based optical fiber, and the angle xcex81 may be of 4 or more degrees.
The first optical fiber may be a chalcogenide optical glass fiber, the second optical fiber may be a silica-based optical fiber, and the angle xcex81 may be of 8 or more degrees.
In a twenty-fourth aspect of the present invention, there is provided a light source comprising:
an optical amplification medium which is one selected from a group of an erbium-doped tellurite optical fiber and an optical waveguide; and
an optical coupler arranged on an end of the optical amplification medium, wherein
at least one terminal of the optical coupler is equipped with a reflector.
Here, the erbium-doped tellurite optical fiber or the optical waveguide may consist of the novel tellurite glasses described above.
The reflector may be comprised of one selected from a group of a dielectric-multiple-film filter and a fiber-bragg-grating.
In a twenty-fourth aspect of the present invention, there is provided an optical amplifier using an erbium-doped tellurite optical fiber or an optical waveguide as an optical amplification medium, comprising
an optical coupler arranged on an end of the optical amplification medium, wherein
at least one terminal of the optical coupler is equipped with a reflector.
Here, the erbium-doped tellurite optical fiber or the optical waveguide may consist of the novel tellurite glasses described above.
The reflector may be comprised of one selected from a group of a dielectric-multiple-film filter and a fiber-bragg-grating.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.